fix: correct error messages when importing courses

[johwuy]

Summary

• Added error handling when making query to Zotcourse.
• Corrected the message that when importing by Study List

Test Plan

• Try to import when text field is empty.
• Try to import when text field has invalid input.

Issues

Closes #872

Future Followup

• import.tsx could be refactored to make the handling of conditionals clearer when importing via Study List or Zot Course, as there is a conditional explicitly handling Zot Course but not Study List."

[johwuy]

I don't have the ability to merge pull requests because I don't have write access.